Tremolo producing means for an electrical musical instrument



March 27, 1962 R. J. ZIEHLKE 3,026,758

TREMOLO PRODUCING MEANS FOR AN ELECTRICAL MUSICAL INSTRUMENT Filed June50, 1958 INV EN TORI ROBERT J, ZIEH LKE United States Patent ice3,026,758 TREMOLO PRODUCING MEANS FOR AN ELECTRICAL MUSICAL INSTRUMENTRobert J. Ziehlke, Janesville, Wis., assignor to Gibbs Manufacturing 8:Research Corporation, a corporation of Wisconsin Filed June 30, 1958,Scr. No. 745,564 3 Claims. (Cl. 841.25)

This invention is concerned with an electronic musical instrument andmore particularly with tremolo producing circuits for an electronicmusicalinstrument.

Some musical instruments, as for example organs, are provided with meansfor producing'a tremolo efiect, i.e. an efiect in which the intensityoramplitude of the tone is varied at a relatively rapid rate as comparedwith variations eifected by the swell or volume control of theinstrument. In pipe organs, this efiect is sometimes achieved by rapidlyvarying the pressure of the air supplied to the pipes. The presentinvention is concerned with a circuit by which a similar elfect isachieved in an electronic musical instrument such as an electronicorgan.

A principal object of the present invention is the provision in' anelectrical musical instrument having means for producing bass and treblemusical tone signals of an improved tremolo producing circuit in whichthe internal cathode-to-anode resistance of an electron discharge tubeis varied at a desired tremolo rate to effect a corresponding variationin the amplitude of the bass and treble musical tone signals.

Another object is the provision in the musical instrument of thepreceding object of means reducing the tremolo efiect on the lower basssignals.

Another object is the provision in the musical instrument of thepreceding objects of a feedback circuit having a time constant longerthan the period of the lowest tone signal and shorter than the period ofthe tremolo rate for minimizing noise in the tremolo producing circuitand assuring a substantially sinusoidal variation in the inter nalcathode-to-anode resistance.

The various features and advantages of the invention will readily beapparent from the following specification and from the drawing, inwhich:

The FIGURE represents a schematic circuit diagram of an embodiment ofthe invention.

The tremolo circuit which forms the basis of this invention is designedfor use in an electronic organ, i.e. an organ in which electricalsignals representing the various tones are generated, amplified andbroadcast through speakers. It will be apparent from a consideration ofthe following description that the present invention may be utilizedwith any musical instrument which produces tone representing signals,regardless of the manner in which they are initially produced, or thenature of their end use.

Turning now to the drawing, an embodiment of the invention will bedescribed in detail. During the course of the description, variouscomponents will be assigned specific values or identified, as byparticular electron tube type designations. It is to be understood thatthese values and type designations are intended to disclose an operativecircuit and are not to be construed as critical, except wherespecifically stated to the contrary. Many alterations and modificationsof the specific circuit disclosed will readily be apparent to thoseskilled in the art.

Briefly, electrical signals representing musical tones are produced bytone generator 1, which may be of any suitable type. The musical tonesignals produced by the tone generator are coupled to a tremoloproducing circuit indicated generally as 2, a tone modifying circuit 3,a voltage amplifier 4, driver stage 5, power amplifier 6 and speaker 7.The tremolo producing circuit 2 includes an amplifier 8 which forms a"variable impedance which 3,026,758 Patented Mar. 27, 1962 is shuntedacross the output of the tone generator, and an audio oscillator 9 whichprovides a control signal for the amplifier, varying the impedancethereof at an audio rate, and thus varying the impedance of the tonegenerator output at the same rate. This causes a corresponding variationin the amplitude of the tone signals at the audio rate of oscillator 9.p

More particularly, the tone signals from the tone generator 1 arecoupled through a D.C. blocking capacitor 15, 0.1 ,uf. (microfarad) to aload impedance com-prising a potentiometer 16, 500,000 ohms, having aterminal 16b connected to a'referen'ce or ground potential 19 and aseries resistor 17, 560,000 ohms, connected to the potentiometer by thetap 16a and having a terminal 17a. This arrangement permits adjustmentof the output level of the tone generator. The tremolo producing circuit2 is connected across the output terminals 16a and 17a.

The terminal 17a is connected by contacts 20a of a jack 20 to a seriesresistor 21, 560,000 ohms which in turn is connected to a shunt bassboost circuit 3. The bass boost circuit comprises a pair of resistors22, 56,000 ohms and 23, 390,000 ohms, resistor 23 being shunted bycapacitor 24, 0.02 ,uf.

The modified tone signal is coupled to the grid 25a of amplifier tube25, a 12AX7. Self-bias for the amplifier is provided by unbypassedcathode resistor 26, 2700 ohms which is returned to ground 19. The anode25b of the amplifier is connected through load resistor 27, 220,000 ohmsand a decoupling circuit including dropping resistor 28, 3,300 ohms andcapacitor 29, 15 ,uf, with a 13+ supply of 320 volts. The amplifiedoutput of voltage amplifier 4 is coupled through a blocking capacitor30, 0.01 f, with a control grid 31a of output tube 31, a 12AT7. Theanode 31b of driver stage 5 is connected directly to 13+ through thedecoupling network made up of resistor 28 and capacitor 29. The outputsignal from the stage is developed across cathode load resistor 32,33,000 ohms, while an unbypassed cathode resistor 33 provides self-bias.Control grid 31a is returned to the junction between resistors 31 and33, by resistor 34, 220,000 ohms.

Returning now to the tremolo producing circuit 2, the amplifier 8includes a triode 40, a 12AT7, having its anode 40b through a blockingcapacitor 41, 0.025 ,uf, to the terminal 17a. The cathode 40a of thetriode 40 is maintained at positive potential by a voltage dividerincluding resistor 42, 150,000 ohms and adjustable resistor 43, 10,000ohms. Adjustable resistor 43 shunted by capacitor 44, 4 i, connects thecathode 40a to the terminal 16a. Anode 40b is connected through loadresistor 45, 330,000 ohms and decoupling circuits including resistor 46,10,000 ohms, capacitor 46a, 15 [.tf., resistor 28, capacitor 29 to theB+ supply. The control grid 40c of the triode 40 is returned to ground19 through a pair of series connected resistors 47 and 48, 1 megohmeach. Inverse feedback for the amplifier 8 is secured by means ofcapacitor 49, 0.01 ,uf. connected between the anode 40b and the controlgrid 40c thereof.

The internal cathode-to-plate resistance of the triode 40 'is connectedin parallel with the impedance of the grid circuit, made up of capacitor49 and resistors 47 and 48 and varies in accordance with the amplitudeof the current passing therethrough. With the tremolo circuitinoperative, this current is adjusted to a median value by propersetting of adjustable resistor 43 in the cathode circuit of theamplifying device.

In order to produce a tremolo effect, it is desired to vary theimpedance, that is, the internal cathode-to-plate resistance, of thetriode 40 above and below its impedance with the tremolo circuitinoperative, at a subaudio rate, thus varying the amplitude of the tonerepresenting signal at the terminals 16a and 17a above and below itsnormal 3 value and at the same rate. Subaudio oscillator 9 includes atriode 50, a 12AX7, the anode of which is connected with 13+ throughresistor 51, 150,000 ohms, resistors 46 and 28. The cathode 50b isreturned to ground through adjustable resistor 52, 10,000 ohms, shuntedby capacitor 53, 4 ,uf. A resistance-capacitance feedback networkconnected to the oscillator 50 includes resistor 54, 3.3 megohms,capacitor 55, 0.01 ,uf., resistor 56, 680,- 000 ohms, capacitor 57, 0.01,uf., resistor 58, 470,000 ohms, capacitor 59, 0.01 ,uf. and capacitor60, 0.01 pf. Feedback from the anode 50ais made to the juncture betweencapacitors 59 and 60. The subaudio frequency 'output of oscillator 50 isobtained across resistor 61,

10,000 ohms, connected in parallel with cathode resistor 52. Blockingcapacitor 62, 4 ,uf., prevents direct current from flowing through loadresistor'61. The audio output of oscillator 50 is coupled throughcapacitor 63, 0.1 ,uf., to the juncture between resistors 47 and 48 ofthe grid circuit of tube 40.

The frequency of oscillation of oscillator 9 depends upon the values ofresistors and capacitors in the feedback network. With the figuresgiven, the oscillator operates at four or five cycles per second. Thevalue of resistor 56 rnaybe altered to change this frequency, althoughfor proper tremolo efiect the frequency should be kept in the range offour to eight cycles per second. The amplitude of the control signalcoupled to gate tube 40 may be varied by adjustment of variable resistor52 in the cathode circuit of the oscillator. For a heavy tremolo effect,this resistor should be set to produce an amplitude variation of thetone representing signals in the signal channel of plus and minus 3 db.The operation of the tremolo circuit is controlled by on-olf switch 64in the cathode circuit of the oscillator, which shorts resistor 61 whenin the off position.

Transient elfects in the tremolo circuit are reduced by the feedbackcapacitor 49 connected between the anode .40b and control grid'40c ofvariable impedance device 40.

The time constant of this circuit, which'includes the grid resistors 47and 48, is selected to be greater than the period of the lowest audiofrequency signals handled by the channel (16 cycles per second) andsmaller than the period of the tremulant control signal from oscillator50. The value of capacitor 41 (0.025 ,uf.) is such that it has anappreciably higher reactance at the lower audio frequencies than at thehigh frequencies, reducing the tremolo effect on the lower bassfrequencies of the tone representing signal.

The heavy feedback provided by capacitor 49 also compensates for thenon-linear characteristic of the gating device 40,.keeping the variationin impedance, which. produces .the'tremolo, substantially sinusoidal atthe anode 40b of the-gate circuit.

The signal appearing across load resistor 32 of output stage 5 isconnected through acoupling transformer 65 with a power amplifier 6.Capacitor 66, connected in series with the primary winding 65a of thecoupling transformer, is selected to tune the primary circuit of thetransformer to series resonance at a frequency of about thirty-twocycles per second. Thus, the load on output stage'31 acts as a high passfilter, eliminating the tremulant frequency from, the signal coupled tothe power amplifier 6 Without affecting the desired tone representingsignals.

While I have shown and described certain embodiments of my invention, itis to be understood that it is capable of many modifications. Changestherefore, in the construction and arrangement may be made withoutdeparting from the spirit and scopeof the invention as disclosed in theappended claims.

I claim:

1. In an electrical musical instrument having a tone 7 comprising anamplifier including an electron discharge tube having an anode, acathode and a grid, a coupling capacitor connecting the anode directlyto one of the terminals, a parallel connected capacitor and firstresistor connecting the cathode to the other terminal, a second resistorconnecting the grid to the other terminal, and an oscillator producingsignals at a desired tremolo rate and including a third resistor acrosswhich the tremolo signals are obtained, the third resistor beingconnected across the second resistor thereby applying the tremolosignals to the gridto vary the internal cathode-to-anode resistance ofthe tube at the tremolo rate, whereby the amplitude of the bass andtreble signals at the output terminals of the load impedance are variedat the tremolo rate.

2. In an electrical musical instrumenthaving a tone generator producingbass and treble signals and having a load impedance connected to thetone generator and including output terminals, a tremolo producing meanscomprising an amplifier including an electron discharge tube having ananode, acathode and a grid, a coupling capacitor'connecting the anodedirectly to one of the terminals, a parallel connected capacitor andfirst resistor connecting the cathode to the other terminal, a secondresistor connecting the grid to the other terminal, and an oscillatorproducingsignals at a desiredtremolo rate and including a third resistoracross which the tremolo signals are obtained, the third resistorbein'g'connected across the second resistor'thereby applying the tremologenerator producing bass and treble signals and having.

a load impedance connected to the tone generator and including outputterminals, a tremolo producing means comprising an amplifier includingan electron discharge tube having an anode, a cathode and a grid, acoupling capacitor connecting the anode directly to one of theterminals, a parallel connected capacitor and first resistor connectingthe cathode to the other terminal, second and third resistors connectingthe grid to the other terminal;

a feedback capacitor connecting the anode directly to the grid, thefeedback capacitor and the second and third resistors having a timeconstant which is longer than the period of the lowest pitch basssignals and shorter than the period of the tremolo signals, and anoscillator producing signals at a desired tremolo rate and including afourth resistor across which the tremolo signals are obtained, thefourth resistor being connected across the third resistor therebyapplying the tremolo signals to the grid to vary the internalcathode-to-anode resistance of the tube at the tremolo rate, wherebytheamplitude of the bass and treble signals at the output terminals of theload impedance are varied at the tremolo rate.

References Cited in the file of this patent UNITED STATES PATENTS2,138,500 Miessner Nov. 29, 1938 2,147,948 Kent et al. Feb. 21, 19392,221,188 Hammond et al. Nov. 12, 1940 2,322,884 Roetken June 29, 19432,485,538 Rowe Oct. 18, 1949 2,534,342 Daniel Dec. 19, 1950 2,817,708Fender Dec. 24, 1957 2,835,814 7 Dorf May 20, 1958 2,835,870 Fretz May20, 1958 2,892,373 Bauer June 30, 1959

